The development of new, cost-competitive processes that utilize renewable resources as feedstocks is vital for a sustainable economy. These processes also represent important milestones toward the goal of reducing the United States' dependence on foreign oil. Introduction of such processes not only avoids the use of more petroleum, but also has the potential to provide substantial energy savings, and reduce greenhouse gas emissions. Although biobased syntheses of certain commercially significant compounds such as 1,3-propanediol have been reported,1 there are comparatively few reported approaches2 to compounds related to terephthalic acid.
The Diels-Alder reaction of pyrones such as coumalic esters with activated alkynes has good literature precedent.3,4 As shown in Scheme 1, the reaction with methyl coumalate (1) involves a cycloaddition to produce bicyclo[2.2.2]octadiene intermediate 2 that loses carbon dioxide to directly form the substituted benzene. Delaney, et al., have utilized this reaction to produce phenols.5 The reaction of activated alkenes, such as acrylates or acrylonitrile, with methyl coumalate can produce a bicyclic lactone 3 that cannot go directly to an aromatic ring by loss of carbon dioxide. One way to construct aromatic rings via this intermediate is to dehydrogenate adduct 3 under conditions that lead to loss of carbon dioxide.6
However, a need exists for methods to condense coumalates with unactivated (electron-rich) alkenes.